The present invention for detecting ultrasonic displacements includes a detection laser to generate a first pulsed laser beam to generate the ultrasonic surface displacements on a surface of the target. A second pulsed laser beam to detect the ultrasonic surface displacements on a surface of the target. collection optics to collect phase modulated light from the first pulsed laser beam either reflected or scattered by the target. An interferometer which processes the phase modulated light and generate at least one output signal. A processor that processes the at least one output signal to obtain data representative of the ultrasonic surface displacements at the target.
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14. An interferometeric apparatus comprising:
a first cavity having a first confocal lens structure; a second cavity having a second confocal lens structure; a device for dividing incoming de-polarized light into a first polarized light component and a second polarized light component, said device also directing said first and second polarized light components into the first and second cavities; a control system to adjust said first and second cavities such that a ratio of light transmitted through each cavity to light reflected back through each cavity remains substantially constant.
4. An interferometeric apparatus for measuring light, comprising:
a first cavity having a first confocal lens structure; a second cavity having a second confocal lens structure; a polarized beam splitting assembly to divide incoming de-polarized light into a first polarized light component and a second polarized light component wherein said polarized beam splitting assembly directing said first and second polarized light components into the first and second cavities; a first detector positioned to detect a first amount of light transmitted through the first confocal lens structure wherein said first amount being represented by variable VT1; a second detector positioned to detect a second amount of light transmitted through the second confocal lens structure wherein said second amount being represented by variable VT2; a third detector positioned to detect a first amount of light reflected back through the first confocal lens structure wherein said first amount being represented by variable VR1; a fourth detector positioned to detect a second amount of light reflected back through the second confocal lens structure wherein said second amount being represented by variable VR2; a first control system to adjust and tune the first cavity to adjust VR1 relative to VT1; a second control system to adjust and tune the second cavity to adjust VR2 relative to VT2.
1. An optical interferometeric apparatus for measuring incoming de-polarized light comprising:
a first cavity having a first confocal lens structure; a second cavity having a second confocal lens structure; a beam splitter for dividing incoming de-polarized light into a first polarized light component and a second polarized light component and for directing said first and second polarized light components into the first and second cavities; a first collection optics for collecting light transmitted through the first confocal lens structure wherein said collected light being represented by variable VT1; a second collection optics for collecting light reflected back through the first confocal lens structure wherein said collected light being represented by variable VR1; a third collection optics for collecting light transmitted through the second confocal lens structure wherein said collected light being represented by variable VT2; a fourth collection optics for collecting light reflected back through the first confocal lens structure wherein said collected light being represented by variable VT2; a control system for adjusting the first cavity to vary an amount of light transmitted through the first cavity in relationship to an amount of light reflected back through the first cavity; and a second control system for adjusting the second cavity to vary an amount of light transmitted through the second cavity in relationship to an amount of light reflected back through the second cavity.
13. An interferometeric apparatus for measuring light, comprising:
a first cavity having a first confocal lens structure; a second cavity having a second confocal lens structure providing a means for dividing incoming de-polarized light into a first polarized light component and a second polarized light component and for directing said first and second polarized light components into the first and second cavities providing a plurality of detectors for measuring the amount of light transmitted through the first confocal lens structure relative to the amount of light reflected back through the first confocal lens structure providing a relationship which is expressed by formula (VR1/(VR1+VT1)); a detector for measuring the amount of light transmitted through the second confocal lens structure relative to the amount of light reflected back through the second confocal lens structure providing a relationship which is expressed by formula (VR2/(VR2+VT2)); and a control system for adjusting the first and second cavities to maintain the following relationship despite a variation in intensity of the incoming de-polarized light:
wherein the constant is a real number between 0.5 and 1∅
2. The optical interferometer of
a first detector to quantify VT1; a second detector to quantify VR1; a third detector to quantify VT2; and a fourth detector to quantify VR2.
3. The apparatus of
wherein the constant is a real number between 0.5 and 1∅
5. The apparatus of
wherein the constant is a real number between 0.5 and 1∅
6. The apparatus of
7. The apparatus of
a first partially reflective spherical mirror; a second partially reflective spherical mirror; each of said partially reflective spherical mirrors having a curvature of radius R1; said first and second partially reflective spherical mirrors facing each other and being spaced from each other by a distance approximately equal to R1.
8. The apparatus of
9. The apparatus of
10. The apparatus of
11. The apparatus of
12. The apparatus of
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This application claims the benefit of U.S. Provisional Application No. 60/091,240 filed on Jun. 30, 1998. Additionally, this application incorporates by reference the prior of U.S. Provisional Application No. 60/091,229 filed on Jun. 30, 1998 entitled "METHOD AND APPARATUS FOR DETECTING ULTRASONIC SURFACE DISPLACEMENTS USING POST-COLLECTION OPTICAL AMPLIFICATION" to Thomas E. Drake.
The present invention relates generally to an apparatus and method of non-destructive evaluation of materials, and more particularly, to an apparatus and method of processing optical information to detect ultrasonic surface displacements through the use of at least one laser to perform a non-destructive evaluation of a material.
In recent years, the use of advanced composite structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries. While composite materials offer significant improvements in performance, they require strict quality control procedures in the manufacturing processes. Specifically, non-destructive evaluation ("NDE") methods are required to assess the structural integrity of composite structures, for example, to detect inclusions, delaminations and porosities. Conventional NDE methods, however, are very slow, labor-intensive, and costly. As a result, testing procedures adversely increase the manufacturing costs associated with composite structures.
Various methods and apparatuses have been proposed to assess the structural integrity of composite structures. One method to generate and detect ultrasound using lasers is disclosed in U.S. Pat. No. 5,608,166, issued Mar. 4, 1997, to Monchalin et al. (the "'166 Patent"). The '166 Patent discloses the use of a first modulated, pulsed laser beam for generating ultrasound on a work piece and a second pulsed laser beam for detecting the ultrasound. Phase modulated light from the second laser beam is then demodulated to obtain a signal representative of the ultrasonic motion at the surface of the work piece. A disadvantage associated with this approach is that the first pulsed laser beam must be modulated. Other U.S. Patents issued to Monchalin et al. and relating to the subject matter of ultrasonic material testing include the following:
U.S. Pat. No. | Title | Issue Date |
5,608,166 | Generation and Detection of | Mar. 4, 1997 |
Ultrasound with Long Pulse | ||
Lasers | ||
4,966,459 | Broadbank Optical Detection | Oct. 30, 1990 |
of Transient Motion from a | ||
Scattering Surface | ||
5,131,748 | Broadbank Optical Detection | Jul. 21, 1992 |
of Transient Motion from a | ||
Scattering Surface by Two- | ||
Wave Mixing in a | ||
Photorefractive Crystal | ||
5,402,235 | Imaging of Ultrasonic- | Mar. 29, 1995 |
Surface Motion by Optical | ||
Multiplexing | ||
4,633,715 | Laser Heterodyne | Jan. 6, 1987 |
Interferometric Method and | ||
Apparatus for Measuring | ||
Ultrasonic Displacements | ||
5,080,491 | Laser Optical Ultrasound | Jan. 14, 1992 |
Detection Using Two | ||
Interferometer Apparatuses | ||
5,137,361 | Optical Detection of a | Aug. 11, 1992 |
Surface Motion of an Object | ||
Using a Stabilized | ||
Interferometric Cavity | ||
4,426,155 | Method and Apparatus for the | Jan. 17, 1984 |
Interferometric Wavelength | ||
Measurement of Frequency | ||
Tunable C. W. Lasers | ||
5,608,166 | Generation and Detection of | Mar. 4, 1997 |
Ultrasound with Long Pulse | ||
Lasers | ||
4,820,981 | Method and Apparatus for | Apr. 11, 1989 |
Measuring Magnetic Losses in | ||
Ferromagnetic Materials | ||
Based on Temperature | ||
Modulation Measurements | ||
4,659,224 | Optical Interferometric | Apr. 21, 1987 |
Reception of Ultrasonic | ||
Energy | ||
4,607,341 | Device for Determining | Aug. 19, 1986 |
Properties of Materials from | ||
a Measurement of Ultrasonic | ||
Absorption | ||
Although these patents describe operable techniques for optically detecting transient motion from a scattering surface, which techniques are useful for ultrasonic composite materials non-destructive test and evaluation, these techniques have numerous failings.
To begin, none of the Monchalin and other known techniques provide the ability to perform with high signal-to-noise-ratios (SNR) at large distances from typically very dark composite materials using small aperture high-speed optical scanning methods. The ability to operate in such a mode has the distinct advantage of increasing the optical scan area coverage and providing substantially improved depth-of-field thereby eliminating the need for active focusing mechanisms.
Other known techniques do not posses the desirable feature of removing common-mode noise from the laser signals using a fully self-referenced interferometric configuration that uses all of the available light without the use of separate stabilization measurements.
Another limitation associated with the Monchalin and other known apparatuses relates to their inability to operate at very high scan rates and process ultrasonic data in real-time. This limitation makes such apparatuses only marginally useful for testing and evaluating composite materials.
Other limitations associated with existing apparatuses relate to general inflexibility of such apparatuses, which may hold all distances low, result in small depth of field performance and only minimal extraction of information from the back scattered signals. These limitations make industrial application of the ultrasonic testing method generally impractical.
The present invention provides an apparatus and method for generating and detecting ultrasonic surface displacements on a remote target that substantially eliminates or reduces disadvantages and problems associated with previously developed laser ultrasonic systems and methods.
More specifically, the present invention provides a method and system for generating and detecting ultrasonic surface displacements on a remote target. The system includes a first pulsed laser to generate a first pulsed laser beam. The first pulsed laser beam produces ultrasonic surface displacements on a surface of the remote target. A second pulsed laser generates a second pulsed laser beam coaxial with said first pulsed laser beam to detect the ultrasonic surface displacements on the surface of the remote target. Collection optics to collect phase modulated light from the second pulsed laser beam either reflected or scattered by the remote target and optionally optically processed to increase the light intensity. An interferometer to process the phase modulated light and generate at least one output signal. A processor for processing the at least one output signal obtains data representative of the ultrasonic surface displacements on the surface of the remote target.
In another embodiment, a method for ultrasonic laser testing in accordance with the invention comprises using a first pulsed laser beam to generate ultrasonic surface displacements on a surface of a remote target. A second pulsed laser beam coaxial is used with the first pulsed laser beam to detect the ultrasonic surface displacements on the surface of the remote target collecting phase modulated light from the second pulse laser beam either reflected or scattered by the remote target also occurs, processing the phase modulated light to obtain data representative of the ultrasonic surface displacements on the surface of the remote target.
A technical advantage of the present invention is that a method for ultrasonic laser testing is provided. The personal invention provides rapid, non-contact, and non-destructive inspection techniques that can be applied to complex composite structures. The present invention provides a flexible, accurate and cost effective method for inspecting complex composite structures. The present invention is able to rapidly scan and test large-sized composite structures. The present invention is able to inspect at angles off normal (i.e., up to ±45 degrees). The present invention does not require expensive fixturing to test composite structures. The present invention does not require the shape of the part to be known prior to testing. The present invention does not require access to both sides of a composite structure to test it for defects.
For a more complete understanding of the present invention, the objects and advantages thereof, reference is now made to the following descriptions taken in connection with the accompanying drawings in which:
FIGS. 14(A and B)are a response analysis and the phase response for a signal that has been modified to permit cancellation of common mode laser noise;
Preferred embodiments of the present invention are illustrated in
The generation laser 210 must be of a frequency that is readily absorbed into the surface of target 150 without causing ablation or breaking down the target material, and it must be of the appropriate pulse duration to induce ultrasonic surface deformations. For example, a transverse-excited atmospheric ("TEA") CO2 laser can be used to produce a 10.6 micron wavelength beam for a 100 nanosecond pulse. The power of the laser must be sufficient to deliver, for example, a 0.25 joule pulse to the target, which may require a 100 watt laser operating at a 400 Hz pulse repetition rate. The generation laser should be absorbed as heat into the target surface thereby causing thermoelastic expansion without ablation.
The detection laser 220 must be of sufficient pulse duration to not induce ultrasonic surface displacements. For example, a Nd:YAG laser can be used. The power of this laser must be sufficient to deliver, for example, a 100 milli-joule, 100 micro-second pulse, which may require a one kilo-watt laser.
As shown in
As shown in
Motorized mirror mount 580 provides a method to redirect the optical path for all of the laser beams beyond optical element 415 but prior to optical scanner 440. Said redirected beams follow a path along a series of reflective turning mirrors 581, 582, 583, 584, 585, and 586 to an internal far-field calibration module 587, the number of turning mirrors is only representative of the desired function, where the actual number could be more or less. Tuning mirror 581, for example, would have an integrated near field adjustable aperture to establish a permanent alignment position to be used in conjunction with the internal far-field calibration module 587. Far-field calibration module 587 is located a distance from optical element 415 to be representative of a typical distance to a target following the standard path through optical scanner 440. Internal far-field calibration and diagnostic module 587 may contain, as example, devices to monitor the power and alignment of each laser, small targets representative of typical testing materials, and devices to assist in the characterization of new materials over a variety of incident angles. As an example, information derived from the internal far-field calibration and diagnostic module 587 could be used to align the generation laser beam 111 to the desired optic axis via motorized reflective tuning mirrors 588 and 589. Such an operation may be necessary to correct for small beam delivery errors created by the remote free-space delivery of beam 111 along the movable axis {x,y,z, theta-1}. Other turning mirrors, not explicitly specified in
The confocal lens structure is designed so that the incoming light falls upon itself after four passes through the cavity. First spherical mirror 875 and second spherical mirror 885 each have the same radius of curvature "r", and when the two mirrors are spaced from each other by this radius "r", the mirrors are said to be in a confocal position, and the light is said to be "re-entrant light" because it falls back upon itself after four passes across the mirrors. First spherical mirror 875 and second spherical mirror 885 are partially transmissive, meaning they pass light as well as reflect light. For example, the said mirrors may be 95% reflective and 57% ignoring absorption and scattering losses, transmissive (i.e. permitting 55% of the light to pass through the mirror).
Some of the incoming light is transmitted through second spherical mirror 885. A third lens 830 focuses the light that is transmitted through second spherical mirror 885 upon transmission-mode detector 890 or optionally an optical fiber attached to transmission mode detector 890, where it can be quantified by variable VT1. Second spherical mirror 885 also reflects a portion of the light back upon first spherical mirror 875, where again, some of the light is passed through spherical mirror 875, and through quarter-wavelength plate 870. When the reflected light passes through quarter-wavelength plate 870 for the second time, the polarization of the light is changed again, and in this case, becomes vertically polarized (s-state). The vertically polarized, reflected light is then rejected by first polarized beam splitter 840 and is reflected upon second lens 820, which focuses the reflected light upon a reflection-mode detector 880 or optionally an optical fiber attached to reflection-mode detector 880, where it can be quantified by variable VR1.
It is possible to vary the amount of light which is transmitted through the cavity relative to the amount of light which is reflected back through the cavity, that is, vary VT1, relative to VR1. One way to vary this relationship is by changing the frequency of the incoming light. An alternative way to vary the relationship is by adjusting the distance between first spherical mirror 875 and second spherical mirror 885. In a confocal relationship, this distance is nominally the radius of curvature "r". One way to vary this distance is to mount at least one of the spherical mirrors on adjustable mounts. In
In
As in the first cavity, first spherical mirror 975 and second spherical mirror 985 each have the same radius of curvature "r" (which is the same radius of curvature as in the first cavity 995), such that when the two mirrors are spaced from each other by this radius "r", the mirrors refocus incoming light upon itself after it travels four passes through the cavities (i.e., two round trips).
The light that is transmitted through second spherical mirror 985 is focused by third lens 930 upon transmission mode detector 990 or optionally an optical fiber attached to transmission-mode detector 990, where it can be quantified by variable VT2. Second spherical mirror 985 also reflects a portion of the light back upon first spherical mirror 975, where again, some of the light is passed through spherical mirror 975, and through quarter-wavelength plate 970. When the reflected light passes through quarter-wavelength plate 970 for the second time, the polarization of the light is changed again, and in this case, becomes horizontally polarized. The horizontally polarized, reflected light then passes through polarized beam splitter 940, and is focused by second lens 920 upon reflection-mode detector 980 or optionally an optical fiber attached to reflection mode detector 980, where the reflected light can be quantified by variable VR2.
It is possible to vary the amount of light which is transmitted or passes through each of the cavities relative to the amount of light which is reflected back through each of said cavities, that is, vary VT1 relative to VR1, and vary VT2 relative to VR2. One way to vary this relationship is by changing the frequency of the incoming light. An alternative way to vary the relationship is by adjusting the distances between first spherical mirrors 875 and 975 and second spherical mirror 885 and 985, respectively. In a confocal relationship, each of these distances is nominally the radius of curvature "r". One way to vary this distance is to mount at least one of each pair of spherical mirrors on adjustable mounts. In
where R is the mirror reflectivity and T is the mirror transmission usually given by T=1-R if absorption and scattering effects are ignored. Now the intensity of the light can be written as:
where epsilon is the change in the cavity length from the confocal length r and lambda is the laser wavelength, and the magnitude operations on the complex function makes the results real expressions. These two equations will produce FIG. 10. Where the reflected light curve 1020 represents the proportion of total light that is reflected back through the cavity, (normalized reflection=VR/(VR+VT)), which figure is always between 0.5 and 1∅ Transmitted light curve 1080 represents the proportion of total light that is transmitted through the cavity, (normalized transmission =VT/(VR+VT)), which figure is always between 0.0 and 0.5. The sum of VR and VT represents the total amount of light that reaches the cavity. Reflected light curve 1020 and transmitted light curve 1080 are each plotted as a function of ε/S, and VR, and VT are both equal when ε/S is equal to 0, 0.25, and 0.5, or more generically, when ε/S=n/4, n being a whole number. Thus it is apparent that the piezoelectric mirror mounts 876 and 976 must move a minimum of λ/4 to provide a sufficient tuning range.
While
wherein the constant η is a real number between 0.0 and 0.5 By fixing the relationship between the reflected light and transmitted light in each cavity in the two cavity designs, the incoming light can be quantitatively processed utilizing the known relationship between the signals of each cavity. A typical operating point would be for η=0.25, thereby 25% of the light would be transmitted through the interferometer and 75% reflected, and would represent an operating point half way along each resonance curve. It is evident from
The present design permits the interferometer to be self-stabilized utilizing exclusively the light which is delivered to the interferometer. Variations in the intensity of the incoming light, which typically are associated with each minute change in positioning of the surface being tested, have little or no impact on the functionality of the interferometer because the signals are based on percentages of light reflected or transmitted with respect to the total amount of light, and thus, are, in effect, normalized for the intensity of the incoming light. It may not be necessary to adjust the cavity tuning position on each laser pulse depending on the drift rate of the laser and the thermal stability of the interferometer. For example at a 400 Hz pulse rate adjustments could be made on every 10th pulse or even less frequently depending on the environment and design. The present invention also uses algorithms based on absolute light intensity to suspend adjustment operations if the light level is too low, thereby preventing erroneous adjustments when the laser beams are off the target and tuning is impossible. When operated in conjunction with pulsed detection lasers it is typical that some form of peak-detection circuitry be employed to hold the peak values constant while a low-speed analog-to-digital converter samples the two or four channels of data. The drive voltage to each of the piezoelectric mirror mounts 876 and 976 is adjusted to compensate for any error based on the previous pulse. Reset of the peak detectors occurs prior to the next pulse. An electo-optic intensity controller (not shown) is typically used to limit the maximum light level sent to lens 810 and subsequently to detectors 880, 890, 980, and 990 thereby preventing damage to the detectors or signal electronics. The information used to control the light level is extracted from the same data used to stabilize the interferometer. Again, based on the results of the prior pulse the appropriate voltage is projected for the next pulse.
In the present invention having a two cavity design, substantially all of the incoming light is utilized for both stabilization and detection. Additionally, the second cavity permits a second set of signals that can be used to improve signal strength.
The output signal from an interferometer in connection with the detection of ultrasonic surface displacements, where the displacement u<<lambda can be represented with the following equation:
where "s" represents the overall signal being produced by the interferometer; "k" is the wavevector defined as
"u" represents the ultrasonic surface displacements being measured (i.e., the desired signal); "r-prime" represents the response function of the interferometer; "a" represents the laser noise (e.g., amplitude fluctuation); "r" represents the response function of the interferometer tb the laser noise, which may be different from the response to an input signal; and "n" represents noise in the detection process (e.g., shot noise, electronic thermal noise, etc.).
The complex response functions of a confocal Fabry-Perot interferometer to an ultrasonic signal with a frequency ωu=2πfu or amplitude noise fluctuation ωn=2πfn can be defined with the following equations:
Where the same substitutions for the beta and gamma functions as defined previously have been used to simplify the equations. In the above equations τ=4rmirror/C is the cavity round trip delay for mirrors of radius "rmirror" and c is the speed of light. Finally the cavity tune position is defined by
where η is the linear tune position between 0 and 0.5 as introduced previously, with 0.25 representing a position halfway along the resonance curve.
If a signal is generated for both the reflected and transmitted components of a confocal cavity, the two signals can be represented as follows:
where, for example, S1 represents the light that has been transmitted through the cavity, and S2 represents the light that has been reflected back through the cavity.
Because the input signal is the same for each formulae, U1=U2 and a1=a2. These relationships are true because the ultrasonic surface displacements and the laser noise are independent of the reflection and transmission modes. Therefore, the two equations can be rewritten as follows:
Ideally, the two modes of the cavity have the same response functions with respect to signal "u", but the responses are negatives of each other because the transmission and reflection modes have opposite response slopes. Hence, r1+r2=0. Ideally, with respect to the laser noise, r1'=r2'. Each response function must be normalized in regard to the tune position of the interferometer and is implicitly corrected to indicate a balanced response between the two modes. For example the if the transmission tune position is at 25% then the reflection is at 75% and a 3× normalization correction factor is used. And, if the signals are used to create a differential, the following relationship results (dropping the time notation):
Dropping the time aspect, for ease of representation, and making the substitutions results in:
Hence, by using a differential signaling scheme, the common mode laser noise can be eliminated. Even if the stated conditions relating to {r1, r2} and {r1', r2'} are not perfectly met, the differential signaling scheme has the effect of removing substantially all common mode noise "a(t)".
Further, if n2 is uncorrelated with n1, then the two noise fluctuations will add incoherently and if the magnitudes of the two are substantially similar, |n1|=|n2|=|n|, we get:
This is the same result one would arrive at by considering the process of averaging two signals with uncorrelated noise terms of equal magnitudes, where the noise will be increase by the square-root of the number of averages and the signal would increase in a linear manner.
1.4 in addition to the removal of common-mode laser noise. Each pair of signals for the two cavities is processed in a substantially similar manner to remove common-mode noise and then the two remaining signals can be further combined yielding another 1.4 increase in SNR.
Typically, the dominate noise source is from laser relaxation oscillations in solid state lasers and is characterized by the "relative intensity noise" or "RIN" of the detection laser. Reduction or elimination of laser RIN is essential for high SNR performance. In systems employing post collection optical amplification schemes, hetrodyne-mixing noise from signal and amplified spontaneous emission ("ASE") can also manifest as a common-mode noise source. Again, use of the self-referenced differential confocal Fabry-Perot interferometer can be used to minimize or eliminate such noise terms.
In another embodiment of the present invention, the two confocal Fabry-Perot cavities can be self-referenced stabilized entirely with light present on the signal detectors such that the corresponding pair of transmitted and reflected intensities represent inverted responses to signals, yet remain in-phase to amplitude noise. This has the added advantage of removing common-mode noise without detailed knowledge of either the signal or noise response functions due to the substantially matched responses between the reflection and transmission for each cavity. Again, the resulting SNR enhanced pair of signals can be further combined to one signal, using the appropriate corrections for optimally combining the processed reflected and transmitted signals.
In one embodiment of the interferometer of the current apparatus, the detectors and the electronic circuitry used to monitor and adjust the cavities are integrated into the interferometer. When the detectors are integrated into the interferometer, there is a potential for introducing noise because the ground plane for the detector circuitry is separate from the ground plane for the data acquisition apparatus, and though the two planes may be connected, the distance between them permits the introduction of unwanted noise.
The output from optical interferometer 1520 is fed to a plurality of detectors 1540 which convert the optical input to analog signaling. The analog signaling is conditioned by analog signal conditioner 1550 and then captured and processed by digital signal processing ("DSP") unit 1560. DSP unit 1560 will compare VR1 relative to Vr1+VT1 and VR2 relative to VR2+VT2 and determine whether adjustments are required in the cavities of optical interferometer 1520 to maintain the desired relationships, as previously discussed. If adjustments are required, then a digital output from DSP unit 1560 may be converted to analog by D/A unit 1570 and sent to electronic controller 1580, which makes the proper adjustments to optical interferometer 1520, for example, by adjusting the piezoelectric devices within the interferometer.
Optical amplifier 1510 functions on a pulse-by-pulse basis, and hence, a trigger signal is used in operation. The trigger signal may be provided indirectly, for example, through power supply 1590, or may be provided directly to optical amplifier 1510. In a real-time like fashion, the electronics process the optical signal to determine the amount of light that has been delivered to the interferometer. If the interferometer is saturated, then the gain is turned down inside optical amplifier 1510, and the interferometer is adjusted to operate in a more optimum range. If the interferometer is operating below an optimum light level, then the gain is turned up inside optical amplifier 1510, and the interferometer is again adjusted to operate in a more optimum range. As illustrated previously in
While circuits other than those illustrated by
Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.
Drake, Jr., Thomas E., Osterkamp, Mark A.
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